To give a feeling of the magnitude of the forces, a hub electric motor with a 12mm axle generating 40 N-m of torque will exert a spreading force of slightly below 1000lb on every single dropout. A torque arm can be a separate piece of metal mounted on the axle which can consider this axle torque and transfer it even more up the frame, therefore relieving the dropout itself from bringing all the stresses.
Tighten the 1/4″ bolt between your axle plate and the arm as snug as possible. If this nut is loose, in that case axle can rotate some volume and the bolt will slide in the slot. Torque Arm china Though it is going to bottom out and prevent further rotation, by enough time this occurs your dropout may previously be damaged.
The tolerances on motor axles can vary from the nominal 10mm. The plate may slide on freely with a little of play, it could go on properly snug, or occasionally a small amount of filing may be essential for the plate to slide on. In scenarios where in fact the axle flats are a little narrower than 10mm and you are feeling play, it is not much of a concern, but you can “preload” the axle plate in a clockwise path as you tighten everything up.
Many dropouts have speedy release “lawyer lips” that come out sideways and prevent the torque plate from sitting flat against the dropout. If this is actually the case, you should be sure to get a washer that suits inside the lip place. We make customized “spacer ‘C’ washer” because of this job, though the lock washer that comes with various hub motors is normally about the proper width and diameter.
For the hose-clamp style, a small length of heat-shrink tubing 
over the stainless band can generate the final installation look even more discrete and protect the paint job from getting scratched. We include several pieces of shrink tube with each torque arm package deal.
However, in high ability devices that generate a whole lot of torque, or in setups with weak dropouts, the forces present may exceed the material durability and pry the dropout open. When that happens, the axle will spin freely, wrapping and severing off the motor cables and potentially causing the wheel to fall right out from the bike.
In most electrical bicycle hub motors, the axle is machined with flats on either side which key into the dropout slot and provide some way of measuring support against rotation. In many cases this is sufficient.